Preliminary understanding of promse

Promse What is it? ？

1. Abstract expression ：Promise yes JS New solution for asynchronous programming in （ I used to use callback Callback function ）
2. Specific expression ：
   （1） Grammatically ：Promise It's a constructor
   （2） functionally ：Promise Object is used to encapsulate an asynchronous operation and get its results

Why use Promise?

One .

1. Specifying callback functions is more flexible
2. You must specify... Before starting an asynchronous task
3. Promise： Start asynchronous task => return Promise object => to Promise Bind callback function , Even after the asynchronous task is finished , Result of callback request
   Two .
   1.Promise Support chain calls , It can solve the problem of callback to hell
   What is callback hell ？
   Multiple asynchronous requests are called in series , And callback , There will be a callback to hell
   When the callback function is nested and called , Not easy to read , It is also not convenient for exception handling

Promise The code of chain call to solve callback hell is as follows ：

// Calls that are not nested relationships , So there will be no problem of callback to hell 
fn1().then((result)=>{
return fn1(result)
})
.then((result2)=>{
return fn2(result2)
})
.then((result3)=>{
 console.log('success')
}).catch(()=>{// Abnormal transmission . Any problem will be transmitted to catch
console.log('error')
})

The perfect solution to callback hell is actually async/await
Pure synchronous coding

async function request(){
try{
const  result = await fn1()
const  result2 = await fn2()
const  result3 = await fn3 ()
console.log(' final result :'+result3)
}catch(error){
console.log('error')
}
}